1. Field of the Invention
This invention relates to isolated dc/dc converters, and more particularly, to the constant-frequency, isolated dc/dc full-bridge converters that operate with ZVS of the primary-side switches in a wide range of input voltage and load current.
2. Description of the Prior Art
The major factors hindering the operation of conventional (xe2x80x9chard-switchedxe2x80x9d) pulse-width-modulated (PWM) converters at higher switching frequencies are circuit parasitics such as semiconductor junction capacitances, transformer leakage inductances, and rectifier reverse recovery. Generally, these parasites introduce additional switching losses and increase component stresses, and, consequently, limit the maximum frequency of operation of xe2x80x9chard-switchedxe2x80x9d converters. To operate converters at higher switching frequencies and, eventually, achieve higher power densities, it is necessary to eliminate, or at least reduce, the detrimental effects of parasitics without a degradation of conversion efficiency. The most effective approach in dealing with parasitics is to incorporate them into the operation of the circuit so that the presence of parasitics does not affect the operation and performance of the circuit. Generally, this incorporation of parasitics can be accomplished by two techniques: the resonant techniques and constant-frequency PWM soft-switching techniques.
The common feature of the resonant techniques is the employment of a resonant tank that is used to shape the current and voltage waveforms of the semiconductor switch (es) to create conditions for either zero-current turn-off, or zero-voltage turn-on. However, zero-current switching (ZCS), or zero-voltage switching (ZVS) in resonant-type converters is achieved at the expense of increased current and/or voltage stresses of semiconductors compared to the stresses in the corresponding xe2x80x9chard-switchedxe2x80x9d topologies. In addition, the majority of resonant topologies need to circulate a significant amount of energy to create ZCS or ZVS conditions, which increases conduction losses. This strong trade-off between the switching-loss savings and increased conduction losses may result in a lower efficiency and/or larger size of a high-frequency resonant-type converter compared to its PWM counterpart operating at a lower frequency. This is often the case in applications with a wide input-voltage range. In addition, variable frequency of operation is often perceived as a disadvantage of resonant converters. As a result, although resonant converters are used in a number of niche applications such as those with pronounced parasitics, the resonant technique has never gain a wide acceptance in the power-supply industry in high-frequency high-power-density applications.
To overcome some of the deficiencies of the resonant converters, primarily increased current stresses and conduction losses, a number of techniques that enable constant-frequency PWM converters to operate with ZVS, or ZCS have been proposed. In these soft-switching PWM converters that posses the PWM-like square-type current and voltage waveforms, lossless turn-off or turn-on of the switch (es) is achieved without a significant increase of the conduction losses. Due a relatively small amount of the circulating energy required to achieve soft switching, which minimizes conduction losses, these converters have potential of attaining high efficiencies at high frequencies.
One of the most popular soft-switched PWM circuit is the soft-switched, full-bridge (FB) PWM converter shown in FIG. 1(a), which is discussed in the article xe2x80x9cDesign Considerations for High-Voltage High-Power Full-Bridge Zero-Voltage-Switched PWM Converter,xe2x80x9d by J. Sabate et al., published in IEEE Applied Power Electronics Conf. (APEC) Proc., pp. 275-284, 1990. This converter features ZVS of the primary switches at a constant switching frequency with a reduced circulating energy. The control of the output voltage at a constant frequency is achieved by the phase-shift technique. In this technique the turn-on of a switch in the Q3-Q4 leg of the bridge is delayed, i.e., phase shifted, with respect to the turn-on instant of the corresponding switch in the Q1-Q2 leg, as shown in FIG. 1(b). If there is no phase-shift between the legs of the bridge, no voltage is applied across the primary of the transformer and, consequently, the output voltage is zero. On the other hand, if the phase shift is 180xc2x0, the maximum volt-second product is applied across the primary winding, which produces the maximum output voltage. In the circuit in FIG. 1(a), the ZVS of the lagging-leg switches Q3 and Q4 is achieved primarily by the energy stored in output filter inductor LF. Since the inductance of LF is relatively large, the energy stored in LF is sufficient to discharge output parasitic capacitances C3 and C4 of switches Q3 and Q4 in the lagging leg and to achieve ZVS even at very light load currents. However, the discharge of the parasitic capacitances C1 and C2 of leading-leg switches Q1 and Q2 is done by the energy stored in leakage inductance LLK of the transformer because during the switching of Q1, or Q2 the transformer primary is shorted by the simultaneous conduction of rectifiers D1 and D2 that carry the output filter inductor current. Since leakage inductance LLK is small, the energy stored in LLK is also small so that ZVS of Q1 and Q2 is hard to achieve even at relatively high output currents. The ZVS range of the leading-leg switches can be extended to lower load currents by intentionally increasing the leakage inductance of the transformer and/or by adding a large external inductance in series with the primary of the transformer. If properly sized, the external inductance can store enough energy to achieve ZVS of the leading-leg switches even at low currents. However, a large external inductance also stores an extremely high energy at the full load, which produces a relatively large circulating energy that adversely affects the stress of the semiconductor components, as well as the conversion efficiency.
In addition, a large inductance in series with the primary of the transformer extends the time that is need for the primary current to change direction from positive to negative, and vice verse. This extended commutation time results in a loss of duty cycle on the secondary of the transformer, which further decreases the conversion efficiency. Namely, to provide full power at the output, the secondary-side duty-cycle loss must be compensated by reducing the turns ratio of the transformer. With a smaller transformer""s turns ratio, the reflected output current into the primary is increased, which increases the primary-side conduction losses. Moreover, since a smaller turns ratio of the transformer increases the voltage stress on the secondary-side rectifiers, the rectifiers with a higher voltage rating that typically have higher conduction losses may be required.
Finally, it should be noted that one of the major limitations of the circuit in FIG. 1(a) is a severe parasitic ringing at the secondary of the transformer during the turn-off of a rectifier. This ringing is cased by the resonance of the rectifier""s junction capacitance with the leakage inductance of the transformer and the external inductance, if any. To control the ringing, a heavy snubber circuit needs to be used on the secondary side, which may significantly lower the conversion efficiency of the circuit.
The ZVS range of the leading-leg switches in the FB ZVS-PWM converter in FIG. 1(a) can be extended to lower load currents without a significant increase of the circulating energy by using a saturable external inductor instead of the linear inductor, as described in the article xe2x80x9cAn Improved Full-Bridge Zero-Voltage-Switched PWM Converter Using a Saturable Inductor,xe2x80x9d by G. Hua et al., published in IEEE Power Electronics Specialists""Conf. Rec., pp. 189-194, 1991, and in U.S. Pat. No. 5,132,889, xe2x80x9cResonant-Transition DC-to-DC Converter,xe2x80x9d by L. J. Hitchcock et. al., issued on Jul. 21, 1992. However, even with the modifications, the performance of these converters is far from optimal.
An FB ZVS-PWM converter that achieves ZVS of the primary switches in the entire load and line range with virtually no loss of secondary-side duty cycle and with minimum circulating energy was described in patent application Ser. No. 09/652,869, filed Aug. 31, 2000 by Jang and Jovanovixc4x87 and assigned to the assignee of this application. This converter, shown in FIG. 2, employs a primary-side coupled inductor to achieve a wide-range ZVS. The two windings of the coupled inductor are connected in series and their common terminal is connected to one end of the primary winding of the transformer, which has the other end of the primary winding connected to the ground. The other two terminals of the coupled inductor are connected to the midpoint of the two bridge legs through a corresponding blocking capacitor. The secondary side can be implemented with any type of the full-wave rectifier such, for example, the full-wave rectifier with a center-tap secondary, the full-wave rectifier with current doubler, or the full-bridge full-wave rectifier. The output voltage regulation in the converter is achieved by employing a constant-frequency phase-shift control as in the circuit in FIG. 1(a).
The circuit in FIG. 2 utilizes the energy stored in the magnetizing inductance of the coupled inductor to discharge the capacitance across the switch that is about to be turned on and, consequently, achieve ZVS. By properly selecting the value of the magnetizing inductance of the coupled inductor, the primary switches in the converter in FIG. 2 can achieve ZVS even at no load. This feature is quite different from the characteristics of the conventional FB ZVS where the capacitances of the lagging-leg switches are discharge by the energy stored in the output filter inductor, whereas the discharge of the capacitances of the leading-leg switches is done by the energy stored in the leakage inductance of the transformer or external inductance. Because in the circuit in FIG. 2 the energy required to create ZVS conditions at light loads does not need to be stored in the leakage inductance, the transformer leakage inductance can be minimized. As a result, the loss of the duty cycle on the secondary-side is minimized, which maximizes the turns ratio of the transformer and, consequently, minimizes the conduction losses. In addition, the minimized leakage inductance of the transformer significantly reduces the secondary-side ringing caused by the resonance between the leakage inductance and junction capacitance of the rectifier, which greatly reduces the power dissipation of a snubber circuit that is usually used to damp the ringing.
In this invention, the concept employed to achieve ZVS of the primary switches in the converter in FIG. 2 is generalized. The generalized concept is used to derive a family of FB ZVS converters with the same characteristics.
The present invention discloses a family of isolated, constant-frequency, phase-shift-modulated FB ZVS-PWM converters that provide ZVS of the bridge switches in a wide range of input voltage and load current. Generally, the converters of this family employ two transformers that are connected to the bridge legs so that a change in the phase shift between the two legs of the bridge increases the volt-second product on the windings of one transformer and decreases the volt-second product on the windings of the other transformer. By connecting a load circuit to the secondary winding(s) of one transformer and by regulating the output of the load circuit, the energy stored in a properly selected magnetizing inductance of the other transformer can be used for creating ZVS conditions. Specifically, as the load current and/or input voltage decreases, the phase shift between the bridge legs changes so that the volt-second product on the windings of the transformer connected to the load also decreases. At the same time, the volt-second product on the windings of the other transformer increases, which increases the energy stored in the magnetizing inductance of the transformer. Therefore, since available energy for ZVS stored in the magnetizing inductance increases as the load current and/or input voltage decreases, the circuits of the present invention can achieve ZVS in a very wide range of input voltage and load current, including no load.
Since the energy used to create the ZVS condition at light loads is not stored in the leakage inductances of the transformer, the transformer""s leakage inductances can be minimized, which also minimizes the duty-cycle loss on the secondary side of the transformer. As a result, the converters of this invention can operate with the largest duty cycle possible, thus minimizing both the conduction loss of the primary switches and voltage stress on the components on the secondary side of the transformer, which improves the conversion efficiency. Moreover, because of the minimized leakage inductances, the secondary-side parasitic ringing caused by a resonance between the leakage inductances and the junction capacitance of the rectifier is also minimized so that the power dissipation of a snubber circuit usually required to damp the ringing is also reduced.
The circuits of the present invention can be either implemented as dc/dc converters, or dc/ac inverters. If implemented as dc/dc converters, any type of the secondary-side rectifier can be employed such, for example, the full-wave rectifier with a center-tap secondary winding, full-wave rectifier with current doubler, or a full-bridge full-wave rectifier. In addition, in some embodiments of the present invention, the transformer that is not connected to the load circuit reduces to a single winding inductor.